1 Field of the Invention
The present invention relates to an air-fuel ratio control apparatus in which a fuel injection valve arranged in an intake passage of an internal combustion engine is pulse-controlled in an on-off manner, and an optimum air-fuel ratio in an air-fuel mixture drawn into the engine is obtained by electronic feedback control correction. More particularly, the present invention relates to an air-fuel ratio control apparatus in which the discharged amounts of nitrogen oxides (NO.sub.x) and incompletely burnt components (CO, HC and the like) are reduced.
2 Description of the Related Art
As representative of the conventional air-fuel ratio electronic control apparatus in an internal combustion engine, there can be mentioned a control apparatus as disclosed in Japanese patent application Laid-Open specification No. 240840/85.
In this type of apparatus, a flow quantity Q of air drawn into the engine and the revolution number N of the engine are detected, and the basic fuel supply quantity Tp (=K.Q/N: where K is a constant) corresponding to the quantity of air drawn into a cylinder is computed. This basic fuel injection quantity is then corrected according to the engine driving states. For example the engine temperature and the like and the air-fuel ratio feedback correction coefficient LAMBDA are determined based on a signal from an oxygen sensor which detects the air-fuel ratio of the air-fuel mixture by detecting the oxygen concentration in the exhaust gas, and correction based on a battery voltage or the like is carried out, and a fuel injection quantity Ti (=Tp.times.COEF.times.LAMBDA+Ts) is finally set.
By sending a driving pulse signal of a pulse width corresponding to the thus set fuel injection quantity Ti to an electromagnetic fuel injection valve at a predetermined timing, a predetermined quantity of fuel is injected and supplied to the engine.
The air-fuel ratio feedback correction coefficient LAMBDA is set to adjust the air-fuel ratio in an air-fuel mixture sucked into the engine to a target air-fuel ratio (the theoretical air-fuel ratio). The LAMBDA is gradually changed in the manner of proportion and integration controls to attain stable, smooth control of the air-fuel ratio feedback. (The proportion control is generally recognized as belonging to the integration control.) The reason for adjusting the air-fuel ratio in the mixture to a value close to the theoretical air-fuel ratio is related to the conversion efficiency (purging efficiency) of a ternary catalyst disposed in the exhaust system to oxidize CO and HC (hydrocarbon) in the exhaust gas and reduce NO.sub.x for purging the exhaust gas. The efficiency of the catalyst is such that the highest effect is attained for an exhaust gas discharged when combustion is performed at the theoretical air-fuel ratio.
Accordingly, a system having a known sensor portion structure as disclosed in Japanese patent application Laid-Open specification No. 204365/83 may be used for the oxygen sensor.
This system comprises a ceramic tube having an oxygen ion-conducting property a platinum catalyst layer for promoting the oxidation reaction of CO and HC in the exhaust gas, which is laminated on the outer surface of the ceramic tube. O.sub.2 left at a low concentration in the vicinity of the platinum catalyst layer on combustion of an air-fuel mixture richer than the theoretical air-fuel ratio is reacted with CO and HC to lower the O.sub.2 concentration substantially to zero. This increases the difference between this reduced O.sub.2 concentration and the O.sub.2 concentration in the open air brought into contact with the inner surface of the ceramic tube, producing a large electromotive force between the inner and outer surfaces of the ceramic tube.
On the other hand, when an air-fuel mixture leaner than the theoretical air-fuel ratio is burnt, high-concentration O.sub.2 and low-concentration CO and HC are present in the exhaust gas. Therefore, even after the reaction of O.sub.2 with CO and HC, excessive O.sub.2 is still present, and the difference of the O.sub.2 concentration between the inner and outer surfaces of the ceramic tube is small, such that no substantial voltage is generated.
The generated electromotive force (output voltage) of the oxygen sensor is characterized in that the electromotive force changes abruptly in the vicinity of the theoretical air-fuel ratio, as pointed out above. This output voltage V.sub.02 is compared with the reference voltage (slice level SL) to judge whether the air-fuel ratio of the air-fuel mixture is richer or leaner than the theoretical air-fuel ratio. For example, in the case where the air-fuel ratio is lean (rich), the air-fuel ratio feedback correction coefficient LAMBDA to be factored into the above-mentioned basic fuel injection quantity Ti is gradually increased (decreased) by a predetermined integration constant, i.e. The feedback control correction constant, whereby the air-fuel ratio is adjusted to a value close to the theoretical air-fuel ratio.
In practice, although the oxygen component in NO.sub.x should be detected as a part of the oxygen concentration in the exhaust gas, this oxygen cannot be detected by the oxygen sensor. Reversion of the electromotive force this tends to occur when the air-fuel ratio is by the oxygen component in NO.sub.x than the theoretical air-fuel ratio. The air-fuel ratio is accordingly controlled to an excessively lean value, whereby reduction of the conversion of NO.sub.x in the ternary catalyst is promoted.
Therefore, reduction of NO.sub.x is attempted by also performing EGR (exhaust gas recycle) control. However, mounting of an EGR apparatus results in increased cost, and the fuel rating is drastically reduced through reduction of the combustion efficiency by introduction of the exhaust gas.
Against this background, there has been proposed an oxygen sensor in which an NO.sub.x -reducing catalyst layer containing rhodium or the like capable of promoting the reduction reaction of NO.sub.x in the exhaust gas is arranged. NO.sub.x is thus reduced, such that oxygen in NO.sub.x can be detected (see E. P. O. 267,764 A2 and E. P. O. 267,765 A2).
If this oxygen sensor is used, the electromotive force of the oxygen sensor is reversed at the true air-fuel ratio. This true air-fuel ratio is shifted to the rich side by the oxygen component in NO.sub.x compared to the theoretical air-fuel ratio at which the electromotive force is reversed when the oxygen sensor has no capacity to reduce NO.sub.x. Accordingly, if this oxygen sensor is used, the air-fuel ratio is shifted to the rich side and adjusted to a value close to the true theoretical air-fuel ratio. Furthermore, since the air-fuel ratio is controlled to a substantially constant level irrespective of the value of the NO.sub.x concentration, the conversions of CO, HC and NO.sub.x are sufficiently increased in the ternary catalyst. The amounts discharged of CO and HC can thus be most effectively reduced and the NO.sub.x content can be effectively lowered, with the result that omission of the EGR apparatus becomes possible.
However, even in the case where the air-fuel ratio is thus controlled to the vicinity of the true theoretical air-fuel ratio, the NO.sub.x, CO and HC (especially NO.sub.x and CO) conversions of the ternary catalyst change abruptly in the vicinity of this value. This is because of the above-mentioned characteristic of the ternary catalyst. The conversion is accordingly unstable because of the dispersion and the deterioration of parts. Since the air-fuel ratio is temporarily made much leaner or richer in the manner of frequency with respect to the theoretical air-fuel ratio, it is difficult to actually obtain high, stable conversions of the catalyst. From the above-mentioned view point, setting the target air-fuel ratio to a slightly leaner value than the theoretical air-fuel ratio would be considered desirable for an engine in which the combustion performance is inherently poor and incompletely burnt components CO and HC are easily formed by incomplete combustion. This is because high, stable conversions of CO and HC in the catalyst can be positively attained while the forming of NO components in the engine is reduced. On the other hand, in an engine in which the combustion performance is inherently good and the NO.sub.x components are easily formed while poor CO and HC components are formed, it would be considered desirable to set the target air-fuel ratio to a value slightly richer than the theoretical air-fuel ratio for attaining the high and stable conversion of NO.sub.x in the ternary catalyst.
Further, even the same engine has different driving states where CO and HC components are easily formed, or where NO.sub.x components are easily formed. Therefore, as in the above discussion, it is preferable to reset the target air-fuel ratio correspond to differences in the engine driving states.
Setting the target air-fuel ratio to slightly richer or leaner value in the air-fuel ratio feedback control should be carried out within a predetermined range of the theoretical air-fuel ratio for effectively reducing the CO, HC and NO.sub.x components in the exhaust gas. If the target air-fuel ratio is set to an extremely lean air-fuel ratio, the amount of CO component exhaust from the engine is reduced with the result that the reduction reaction between NO.sub.x and CO can hardly be performed. As a result the reversing point of the output voltage from the oxygen sensor can not be shift to any richer air-fuel ratio than is the case using the oxygen sensor without the NO.sub.x reducing capacity, and the function of reducing the NO.sub.x component amount using air-fuel ratio feedback control and the oxygen sensor with NO.sub.x reducing capacity is no more effectively performed.
If the target air-fuel ratio is set to an extremely rich air-fuel ratio beyond the predetermined range not only is the amount of CO and HC components increased, but the NO.sub.x reducing reaction in the NO.sub.x reducing oxygen sensor and the ternary catalyst is saturated.
Consequently, the target air-fuel ratio in the air-fuel ratio feedback control apparatus must be set to the optimum value within the predetermined air-fuel ratio range in order to reduce the CO and HC components and also NO.sub.x components when the air-fuel ratio feedback control apparatus includes the NO.sub.x reducing oxygen sensor.